1. Origin of the Invention
The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected not to retain title.
2. Field of the Invention
The present invention is directed to high temperature structural members such as used in arcjets and magnetoplasmadynamic (MPD) thrusters. More specifically, it is directed to a method for cooling such members.
3. Description of the Prior Art
High temperature plasmas such as produced in magnetohydrodynamic generators (MHD), magnetoplasmadynamic (MPD) thrusters and arcjets are highly corrosive to most materials. Consequently, refractory materials such as high temperature ceramics and high melting point metals, including for example tungsten and molybdenum, are used in structural elements that are exposed to the plasma environment.
FIG. 1 is a cross sectional view of a known arcjet 10. The arcjet 10 has an intake section 10a into which an ionizable plasma fluid 11 such as ammonia, argon, helium or hydrogen is injected at a first velocity. The arcjet 10 includes a plasma accelerator section 10b for accelerating the plasma fluid 11 to a substantially higher second velocity A tubular anode 14 made of a refractory metal such as thoriated tungsten is provided in the accelerator section 10b concentrically surrounding a refractory metal cathode 16 which is also composed of a refractory metal such as thoriated tungsten. An insulative propellant injector 15, made of a high temperature dielectric such as boron nitride, separates the cathode 16 from the anode 14. The anode 14 is fitted into a slightly tapered body 12 made of a refractory metal such as molybdenum. A high voltage +HV is applied across a space between the anode 14 and cathode 16 to generate an electric arc 17. The generated arc 17 interacts with the flowing plasma fluid 11 to heat the fluid to temperatures in excess of 1000.degree. C. and to ionize the fluid.
The plasma fluid 11 is injected tangentially in a spiral-like fashion into a plenum chamber 18 of the device to create a vortex 11a about the cathode 16. The vortex 11a is directed into a narrow constriction area 19 of the accelerator section 10b wherein the plasma fluid is super-heated by the electric arc 17. The plasma fluid is then accelerated (primarily by thermal expansion) when it escapes through an expansion nozzle 20 of the device. Charged particles of the ionized plasma may be further accelerated by interaction with a magnetic field oriented orthogonally to the direction of their flow in the expansion nozzle 20. The magnetic field can be supplied from an external flux source outside of the device 10 or it can be generated entirely by the current of the electric arc 17. The combined effects of thermal expansion, generated magnetic fields and the high voltage electric fields between the cathode and anode create an acceleration force which propels the moving plasma 11 in an outflow direction A, as shown, out of the expansion nozzle 20 (defined by the anode 14) to a plasma discharge end 10c of the arcjet engine 10.
Electrons emitted from the cathode 16 are collected at a collecting area 14a on an inner surface of the anode 14 which defines part of the expansion nozzle 20. Because of the high temperatures involved (1000.degree. C. and above), the electrodes 14 and 16 of the arcjet engine are susceptible to destruction from overheating. A variety of factors can lead to undesirable heat build up in localized portions of the engine. Such heat build up should preferably be dissipated as rapidly as possible to avoid the danger of any melting of the engine parts. The anode 14 in particular suffers from a phenomenon known as "anode spot formation" which is characterized by localized melting of various areas at the inner surface of the anode. Anode spot formation is often most pronounced at the collecting area 14a where the electric arc 17 meets with the expansion nozzle's inner wall. The localized melting causes pitting and ultimately leads to the complete destruction of the anode 14.
Active cooling of exterior surfaces of the anode 14 with dynamic cooling devices such as heat pipes and the like has been proposed as a solution for the overheating problem. Active cooling methods are disadvantageous because they require complicated pumping systems for moving a cooling fluid. When such pumping systems are incorporated into the design of an engine they add to the overall mass of the engine and increase the cost of the engine. The present invention provides another means for solving the overheating problem.